String Theory Chapter 2
Why Should Anybody Care? In This Chapter The meaning of theory unification The obvious that’s not so obvious Being open to a conceptual revolution The coexistence of science and religion The way some people describe string theory and similar efforts—as an exercise in abstract mathematics, postulating entities as small in relation to an atom as we are to the known universe—you could be forgiven for wondering why anyone should care. Even theorists themselves have been guilty of downplaying their own theory. However, although the strings or loops may be small, they imply something huge: a radically new view of the world. The Tree of Physics Physics deals with some of the deepest questions one can ask, like: Why does toast always fall with the buttered side down? How can you avoid pocketing a cue ball when playing pool? Why is my desk always such a mess You don’t need to jump straight to big questions such as the nature of space and time. These little mysteries of everyday life lead you to them, since every “because” leads to another “why.” The reasons (the because) could involve Isaac Newton’s laws of motion and gravity or the properties of materials. Then you ask why those laws hold, and the laws beneath them, and so on. As you descend further and further, a remarkable fact about our universe emerges: phenomena that seem completely different have related explanations. People used to think two distinct principles governed the fall of an apple and the cycles of the moon, but Newton traced both to the same basic law of motion. And people used to regard magnetic attraction, static electric shocks, and light beams as unrelated, but the nineteenth-century physicist James Clerk Maxwell showed they are aspects of one single force called electromagnetism. Nature is like a big, bushy oak tree. If you start at one of the leaves—the observable phenomenon—and follow the twig to the branch to the limb, you’ll discover they link together. No matter which leaf you start with, you’ll eventually wind up on one of two main boughs: Albert Einstein’s general theory of relativity, which deals with gravity; and quantum theory, which accounts for chemical and nuclear reactions. The two not only handle different types of questions but also handle them in different ways. Relativity gives answers in terms of the behavior of space and time; quantum theory speaks of the interactions of subatomic particles. By the time you reach one of these two theories, you have transformed the original mundane question into a deep one, which can be defined as one that a five-year-old would ask and the greatest expert in the world couldn’t answer (never mind the parents). What are space and time? What are particles, and why do they interact as they do? The physicists now working to develop string theory and other such theories used to be the five-year-olds who never got a satisfying answer to these questions and have been bothered about it ever since. Their ambition is to find the link between relativity and quantum theory and reach the trunk of the tree, the place where all possible questions about the physical world converge. Take, for example, the falling toast question. As the toast slides off the kitchen counter, its weight becomes imbalanced, so it does a somersault as it falls—a consequence, ultimately, of general relativity. But at the rate it spins, it can only complete half a turn by the time it hits the floor because of the countertop height that human anatomy entails—a consequence, ultimately, of quantum theory. And here the chain of reasoning stops. Simply put, the universe is built in such a way as to ensure that toast dropped from the typical table or counter falls face down, but no one knows why. So forget about exotica such as black holes and the big bang; explaining the mess on your kitchen floor is reason enough to unite general relativity and quantum theory. The Joy of Unification If you didn’t know much about trees, you might try to make one by taking two sticks and wrapping a rubber band around them. Ta-da: you have a tree. That’s like trying to make a unified theory of physics by taking the various observations you’ve made and drawing one big box around them. Ta-da: you have a unified theory. But somehow that doesn’t seem very satisfying. If you describe nature as disjointed phenomena, you can’t claim to know how it or each of its components ticks. The tree of physics is more than a bunch of unrelated branches because it shows the system of how one aspect of nature follows organically from another. The thing has a life of its own, and surprises await the patient explorer. There’s no better place to see the underlying order of nature than on a pool table. The three main principles of motion, the conservation of energy, momentum, and—if the ball is spinning—angular momentum, come into sharp focus as you hit the ball. Conservation of energy means that an object or collection of objects doesn’t gain or lose energy over time. Conservation of momentum or of angular momentum means that a moving or spinning object continues to move or spin unless some outside force acts to stop it. Pool balls may slow down, but that just means the energy of their motion is converted into other forms, such as heat and sound. The total energy doesn’t change. Much the same holds for momentum and angular momentum. When balls collide, they share their motion or spin but do not lose it. These conservation principles, in turn, arise because the laws of physics do not vary in space and time. Here, “space” is the surface of the pool table. If it’s level, the ball won’t gain or lose momentum on its own, and if the felt covering the surface is smooth, the ball will retain whatever spin you give it. More abstractly, time is a level surface in that the laws of physics don’t distinguish past from future. If you put these principles together and do some math, you can deduce that a rolling cue ball, after colliding with the target ball, heads off at an angle of about 30 degrees to its original path. (For more detail, see The Complete Idiot’s Guide to Pool and Billiards by David Alciatore.) Make sure the pocket isn’t in that direction at that angle, and you’ll save yourself a lot of embarrassment. If you do scratch, you can always blame it on the universe. Those three principles allow for a huge variety of pool games. Physicists don’t say that a pool game is just a few principles or that the world is just particles, any more than biologists say that the variety of species is just the product of evolution. To the con- trary, modern scientific theories reveal the world as a process of self-creation. From the base of the tree on up, each level adds something that didn’t exist at the lower levels. If that weren’t the case, if the lower levels weren’t any simpler, the laws of phys- ics would need to specify each and every aspect of the world, and where would the creativity be in that? It would be like saying the species all came into existence just as they are and stayed that way, static and passive. The idea of emergence, whereby complex phenomena emerge from simple laws, is often taken to be the opposite of reductionism, which breaks systems down into simpler pieces. It’s more productive to think of reductionism and emergence as two sides of a coin. When you take something apart, you do so not just to catalog the pieces, but also to figure out how they fit together—and then to find new things they might do. So the unity of nature is not a trivial unity or undifferentiated blandness, but rather a unity with countless forms of expression. Emergence is the principle that a complex system has properties its components don’t. In the words of physicist Philip Anderson, “The whole becomes not only more than but very different from the sum of its parts.” Reductionism is the principle that a complex phenomenon can be broken down into smaller pieces that are easier to explain. The ultimate ambition of physics is to push this idea to the max: to show that every- thing we know—even things that we don’t normally think of as “things,” such as forces, space, and time—are all aspects of the same basic stuff, be it a wriggling string or an atom of space. Far from reducing nature to a beige mush, the fully unified theory could reveal that the world is richer than anyone ever imagined. Seldom does a theory bring together what people already knew without opening their eyes to possibilities they’d never suspected. To go back to the tree, once you trace the leaf to the branch to the tree trunk, you can explore a new, different branch. And who knows what you might come across—maybe a three-toed lizard or an abandoned tree fort. Every past unifica- tion in physics has shown that the world we know is just a small part of what’s really out there. Maxwell introduced us to forms of light beyond the range of our vision. Einstein brought us black holes. Although string theorists’ toehold on the tree is still shaky, they’ve already caught glimpses of truly mind-blowing marvels. Great Unifications in Physics Year Unifier(s) Theory What It Unified 4th century b.c. Aristotle Aristotelian Matter, change, natural philosophy motion, and cause 1686–1687 Isaac Newton Laws of motion Celestial and and gravitation terrestrial motion 1861 James Clerk Electromagnetism Electricity, magnetism, Maxwell and light 1869 Dmitri Periodic table Chemistry Mendeleev 1905 Albert Einstein Special theory of Electromagnetism and relativity laws of motion 1915 Albert Einstein General theory of Special relativity and relativity gravitation 1900s–1920s Neils Bohr, Quantum mechanics Electromagnetism and Werner atomic theory of matter Heisenberg, Erwin Schrödinger, and many others 1920s–1940s Paul Dirac, Quantum field Special relativity and Richard theory quantum mechanics Feynman, and many others 1960s–1970s Abdus Salam, Electroweak Electromagnetism and Sheldon theory weak nuclear force Glashow, Steven Weinberg, and many others Why Is This Theory Unlike All Others? I hate to be the one to break the news, but somewhere hidden in your thoughts is an interloper, something you’ve taken for granted all your life, something obvious, essential, and wrong. Scientists, too, have grown up with this assumption all their lives. Part of our shared worldview has got to go, but no one knows which. Smoking out interlopers is one of the creative acts needed for unification. As Einstein was developing his special theory of relativity, he faced a serious dilemma. Newton’s laws of motion were spectacularly successful. So was Maxwell’s theory of electromag- netism. But the two refused to connect. Maxwell’s equations indicated that light trav- eled at a fixed speed, yet Newton’s laws suggested there was no such thing as a fixed speed. If the light source is aboard a moving train or planet, the light should get a boost. So what gives? And this is the essential paradox of unification. To be worthy of unification, a theory has to be successful, but if a theory is successful, what need does it have to unite with another? The predicament is like that of two perfect people who want to marry the perfect spouse. They find each other, and you can practically hear the violins in the background. But because both think of themselves as so perfect, they are unwilling to make the compromises needed to live together. Einstein realized the trouble was something so obvious no one had questioned it: the assumption that time is absolute, passing at the same rate for everyone. As soon as he showed this interloper the door, the two theories clicked. If time slows down as the light source speeds up, then light travels at a constant rate. In this way, science is about undiscovering things as much as discovering them. The same drama is now unfolding again. Both relativity and quantum theory are spec- tacularly successful. Physicists do not know of a single experimental exception to either; both have solid theoretical formulations. Yet they are incompatible. They barely even speak the same language. So what gives? Whatever it is, it has to be profound, or it would have been worked out by now. Physicists have sweated over a quantum theory of gravity for nearly 90 years. It has become a multigenerational project, like the building of a cathedral. Nearly all the top physicists of the past century devoted at least part of their careers to it, and Einstein himself worked on his version of a unified theory for the last third of his life. Talk about delayed gratification! In turn, physicists expect that quantum gravity will be even more revolutionary than past unifications. It may well entail a full unification of all the phenomena known to humans, in which case it will be the first theory without fine print, such as “use only for small particles” or “don’t apply at such-and-such a time.” No one knows whether it will be the “final” theory—one that needs no further explanation—or not. But it will mark the end of the reductionist strategy that has proved so productive in physics: the effort to seek explanations in terms of ever-smaller things. There will simply be no such thing as “smaller.” Space and time themselves might emerge from more funda- mental entities that exist beyond space and time. there is a basic length scale, below which the notion of space (and time) does not make sense, we cannot derive the principles there from deeper principles at shorter distances. Therefore, once we understand how spacetime emerges, we could still look for more basic fundamental laws, but these laws will not operate at shorter distances. This follows from the simple fact that the notion of “shorter distances” will no longer make sense. This might mean the end of standard reductionism. —Nathan Seiberg, Institute for Advanced Study In the Loop I put quotation marks around “final” in “final theory” because, as powerful as the theory might be, it’ll leave plenty of mysteries unanswered—including some of the biggies, such as the nature of consciousness. Physics tells us what the building blocks of nature are, but just because we learn how to make bricks doesn’t mean we know how to build a house. Quantum Leap Such a theory might be the only hope for making sense of our messy desk. The totter- ing piles themselves are easy enough to explain: there are more ways to be disorganized than organized, so the world around us naturally degenerates into chaos unless we expend effort to keep it shipshape. The real mystery of our desk isn’t that it’s messy, but that it ever used to be orderly. The answer ultimately lies in the fact that the cosmos as a whole started off in a nice, clean, crisp condition. Those starting conditions are beyond the scope of current theories; they call not just for a new theory, but for a new sort of theory. To try to keep your desk clean is to fight cosmic destiny. Big Ideas Don’t Like to Be Cooped Up If you carefully watch the positions of the planets night after night, you’ll notice that many of them periodically seem to stop moving, go into reverse, stop again, and then resume their forward motion. Humans watched this happen for thousands of years before someone, most famously Nicolaus Copernicus, realized it meant the sun, rather than Earth, was at the center of the solar system. To some people, swapping two celes- tial bodies sounded like an exercise in pure mathematics. But its implications quickly became apparent. For starters, this repositioning meant our planet was only one among many. It sug- gested that there wasn’t one set of laws that applied to stars and another to those of us stuck on the ground. Instead, a single set of laws governed all the universe. The effects of the conceptual revolution rippled outward into society, feeding the Renaissance and Enlightenment eras. Although astronomy was only one part of this broader movement, and although the seeds for this intellectual flowering had been planted long before Copernicus, the celestial rethink became the prototype for questioning authority of all kinds. As science historian Thomas Kuhn wrote in The Copernican Revolution, “A scientist’s solution of an apparently petty, highly technical problem can on occasion fundamentally alter men’s attitudes toward basic problems of everyday life.” Like individuals, a whole society can grow when forced to confront new ideas. New ideas beget newer ones, and science can play a special role in jumpstarting this virtu- ous cycle. It looks beyond everyday experience or shows the experience in an unex- pected light, so its insights are genuinely new. Consider how far our understanding of physics and astrophysics has come within the lifetime of the oldest people today. At the start of the twentieth century, no one knew molecules or atoms existed, let alone subatomic particles. Most of the electro- magnetic spectrum, ranging from radio waves to x-rays, was a laboratory curiosity. The planets of our solar system were tiny discs of light; no one had ever seen images from the surface of another world. No one had even seen our planet as a planet: a blue marble on black velvet, coated with a frag- ile veneer of water and air. These concepts have led to snazzy gizmos and made a lot of people rich, but just as important, they have made the world that much more interesting a place to live in. We can expect the same from the unification of physics. Even most people who are enthusiastic about string theory tend to underestimate how radical it will prove to be in its impact on how we understand physical law. —Edward Witten, Institute for Advanced Study Sense and Transcendence When I was a kid, I remember playing the game of opposites. I’d tell my friends a word and ask them what its opposite was. By shouting words fast enough, I hoped to catch them saying the opposite of “dog” is “cat” or “salt” is “pepper.” When I ask people for the opposite of “science,” I sometimes catch them saying “religion.” In reality, the two are as opposite as dogs and cats. They might chase each other, but they wouldn’t even know what to do with their supposed adversary if they caught it. If you give them plenty of pillows to nap on, they are usually content to leave each other alone. In an obliging household, they might even be found lounging around together. I am not a religious believer myself, but I have found that some of the most intense discussions I have had about science are with strong believers. They care. The way the world is put together matters to them. They think about it; they reflect on it. So I approach the topic of science and religion with the experience that, in an obliging household, science and religion can be found lounging together. Does science oppose religion? It’s true that science usurps what used to be a major goal of religion: providing explanations for natural phenomena. In so doing, it has made room in our culture for a secular worldview. But that is not the same as saying science proves a secular worldview. Science is agnostic. By its very nature, it is incapable of saying whether a transcendental reality exists or not. Nor can it provide a comprehensive moral code. Scientists who promote atheism are speaking not as sci- entists but as adherents of their own belief system. All Tangled Up Believers and nonbelievers alike can find much to reflect on in modern physics. Many people are struck by a resemblance between the big bang and the Biblical account of Genesis. Others draw a comparison to Kabbalah or Sura 21 of the Koran. Still others see a link between quantum theory and Hindu, Buddhist, or Taoist mysticism. There is more than one way of telling the story of the universe, drawing out the aspects that are meaningful to each of us personally. One runs into trouble only if interpretation goes on in a vacuum, cut off from evolving human understanding, in which case one needs to ask these questions: “What makes me so sure I’m right? What makes me so sure that the universe conforms to my preconceptions?” A unified theory will give us all much to chew over. What if the deepest foundations prove to be beyond reason? Physicists might be unable to come up with a final theory no matter how hard they try, or they might develop a unified theory that seems arbi- trary. Few other branches of science have such potential for clarifying the boundary between physics and metaphysics, where questions of “how” give way to questions of “why.” A Shared Effort To feel the world clicking together in your head is one of the greatest pleasures in life. It’s no surprise scientists become addicted to it. As Jacob Bronowski, the British mathematician and essayist, argued, this pleasure is one that anyone can share, because when you realize what the scientist has done, you live the original eureka moment for yourself. This collective participation in science goes far beyond individual light bulbs going off in individual heads. The search for deeper theories of nature is hard, and people need to pool their talents. And that doesn’t just mean scientists. If you traced the origin of every part in, say, a particle collider, along with the parts that make up those parts, the machines it took to make them, the support the makers of those machines needed, and the money the whole thing required, you would find it took hundreds of millions of people to bring the instrument into the world. The achievement belongs to all of us. The Least You Need to Know String theory and others of its sort try to tie up or unify everything we know into one related package. If physicists are right, things you thought were totally different are actually deeply related. To create a unified theory, scientists need to pinpoint the extraneous assumptions in their current understanding. The theory could change our thinking in ways we can’t anticipate, in the way all truly novel things do The Great Clash of Worldviews String theory and other quantum theories of gravity are the latest stage in a revolution that began early in the twentieth century with Einstein’s theo- ries of relativity and quantum theory. But the word “theories” doesn’t do justice to these intellectual achievements. They’re not just a bunch of geeky equations; they’re entire worldviews—broad-ranging ways of thinking about nature that are based on intuitive ideas yet have deeply counterintuit tive implications. Category:Science